Detection of moisture in gases such as air, that is, humidity, is important in relation to environmental problems. Recently, not only for obtaining more comfortable life space, but also with the advance in home automation and factory automation, detection, measurement, and control of humidity by a sensor becomes more and more important in view of operation environment of precision devices which are subject to troubles due to dew condensation.
Heretofore, various methods have been used for detecting humidity of gases such as air, such as (1) a method utilizing elongation of hair when it absorbs water vapor (so-called hair hygrometer), (2) a method utilizing a change in electrical resistance of an electrolyte by water vapor, (3) a method of detecting a change in resistance or volume of a dielectric material such as organic substances, semiconductors, metal oxides, and the like when moisture is adsorbed by the material, and (4) a method of detecting a weight change of a quartz oscillator coated with polyamide when moisture is adsorbed by the oscillator.
However, with these conventional methods, the method (1) has problems in that the operable ambient temperature is limited to the vicinity of room temperature, and the method cannot be used in the presence of corrosive vapors such as acid or base. In the electrochemical method (2), the device becomes complex in structure as compared to other methods, and requires maintenance of the electrodes and electrolyte. A method utilizing a change in resistance of the method (3) often uses conductivity of H.sup.+ ion generated by adsorption of water, and therefore the resistance of the humidity sensor element becomes excessively high at low humidities where the adsorbed water amount is small, and the resistance and humidity tend to be difficult to be exactly measured by an ordinary method. Further, the device generally has a difficulty of measurement due to high element resistance, and a method using a hydrophilic polymer as a moisture sensitive material cannot be used at high humidities having a possibility of dew condensation.
Further, in the method (4) using a weight change, since a signal is always output when a change in the weight of element occurs even by adsorption of molecules other than water, it requires a devise for selective detection of only water, which is sometimes difficult. Since the humidity detection signal is output as an electrical signal, the humidity detection signal tends to be disturbed by strong electromagnetic waves or noise generated by various emission sources when humidity detection is carried out by a remote control in a wide area such as in a chemical plant or a coal mine. Further, the humidity detection method based on the electrical signal is difficult to be combined directly with an information system or control system using optical signals, which recently becomes widely used, and an apparatus is required for converting the electrical signal to optical signal.
Therefore, development is in demand of a humidity detection apparatus which is immune to electrical disturbance and can be easily combined directly with an optical information system or optical control system. Heretofore, as methods for detecting humidity by an optical signal, (1) a method utilizing humidity dependence of visible light absorption spectrum or fluorescence intensity of a film comprising a polymeric carrier combined with an organic dye such as crystal violet, betaine type dye, or rhodamine type dye, and (2) a method utilizing a change in visible light absorption spectrum of cobalt chloride carried on a polymeric carrier, have been studied. However, organic dyes and organic polymers are low in heat resistance and relatively narrow in the operation temperature range, and tend to undergo an irreversible change if a highly reactive vapor is present in the environment. Therefore, a humidity detection material with improved chemical and thermal stability and a humidity detection method using the material are required.
In view of the above prior art problems, a primary object of the present invention is to provide a humidity detection material which is high in chemical and thermal stability and can detect humidity by an optical means and a humidity detection method using the material.